262 PROCEEDINGS OF THE CALIFORNIA 



the American River, near Sugar Loaf. These gorges, have all, undoubtedly, 

 been occupied by glaciers, and therefore, probably been cut out by glacial 

 agency. The remarkable verticality of the walls seems to be the result of the 

 perpendicular cleavage of the granite in this part. 



C. General Structure of the Sierras. — Mount^iin chains are due to general 

 causes affecting the whole earth ; most probably to the shrinkage of the interior 

 more than the exterior.* But all the smaller inequalities — all that consti- 

 tute scenery — are the result of erosion. Every peak, dome, ridge, caiion, valley^ 

 is due to this cause alone, except where recent volcanic action has occurred. 

 Like many other great chains, the Sierras consist of granite axis and stratified 

 fianks ; but the author believes that granite is but the last term of metamor- 

 phisni of deeply buried sediments subsequently exposed by erosion. That the 

 granite of the Sierra is exposed by erosion, is very clear. The slates originally 

 covered the whole mountain-chain. They still exist on the very crest, because 

 the erosion was less there. On the slopes, ice-erosion has removed the slates 

 and much of the top granite ; yet even here a few patches of slate are left, as on 

 Mount Hoffman. But the slatts do not appear in force until we reach the 

 limits of the ice-erosion, viz., in the foot-hills, where they form the auriferous 

 belt. 



JJ. Ice-erosion and Water-erosion compared. — Eroded forms are determined 

 partly by rock structures and partly by the kind of erosion agent. Forms de- 

 termined by ice are different from those determined by water. Snow-fountains 

 and ice-rivers — or frost disintegrations and glaciers — tend to produce sharp 

 peaks, spires and combs, and broad valleys occupied by lakes and meadows ; 

 while water, on the contrary, tends to produce rounded summits and ridges, and 

 deep V-shaped canons. 



Mr. R. H. Stretch read the followino; relatino; to a certain insect 

 pest, exhibited at a previous meeting : 



Notes on a Species of Coccus? recently found in California. 



BY R. H. STRETCH. 



At a former meeting, certain insects forwarded to this Society from Menlo 

 Park, San Mateo county, by Mr. Gordon, were referred to me for examination. 

 I have the honor to report that I found them to belong to the Homopterous 

 family Coccida, and to be one of that large class of noxious insects popularly 

 known as Scale Bark-lice. As these are among the most troublesome pests 

 with which the orchardist has to contend, I have deemed it desirable to make 

 this report as complete as may be, especially as several species of this family 

 are gaining a foothold in California at the present time ; and unless attention 

 is called to them and measures taken for their extinction, they may become as 

 serious drawbacks to our orange growers, as their allies have proved in Florida 

 and the Sandwich Islands. 



* See papers ou this subject, American Journal for November and December, 1872. 



